A quantity which serves as a reference to accurately define vertical positions. In the United States, two predominant reference surfaces for elevation are the National Geodetic Vertical Datum of 1929 (NGVD 29) and the more recent and more precise North American Vertical Datum of 1988 (see Datum).

The level surface to which elevations are referenced. On topographic maps the vertical datum approximates mean sea level and is based on numerous tidal observations and geodetic survey leveling. (See Mean sea level).

a calculation based on a mean sea level calculated over several years

a geoid that represents the best approximate mean sea level

a level surface of reference (usually mean sea level or the geoid) from which elevations are measured

a base measurement point from which all elevations are determined.